In The Merchant Of Venice what do Antonio and Shylock think of each other?
I think you need look no further than Act I scene 3 to find a full description of how Shylock and Antonio think about each other. In many ways, the central conflict of the play is between these two characters, as Shylock seems to pick Antonio as a representative of all those who have mocked and disparaged his people or tribe, and Antonio seems to pick out Shylock especially for bad treatment because of his race.
Shylock's aside in Act I scene 3 when Antonio enters his office is particularly telling, revealing the reasons for his hatred and dislike:
How like a fawning publican he looks!
I hate him for he is a Christian,
But more for that in low simplicity
He lends out money gratis and brings down
The rate of usance here with us in Venice.
Later on in the same scene, Shylock openly confronts Antonio with the behaviour he has shown Shylock, saying that he has bullied him, insulted him and spat on him. Of course, Shylock is enjoying the twist of fate that has Antonio, his enemy, who has so often mistreated him, come to Shylock begging for money, and is using this incident to point out to Antonio the inhumanity of his treatment of Shylock. But it is clear that Antonio, in his response, feels no remorse:
I am like to call thee so again,
To spit on thee again, to spurn thee too.
If thou wilt lend this money, lend it not
As to thy friends, for when did friendship take
A breed for barren metal of his friend?
But lend it rather to thine enemy,
Who, if he break, thou mayst with better face
Exact the penalty.
Antonio here expresses no guilt, but maintains the existence of the enmity between him and Shylock, basically offering Shylock the chance to gain his revenge if Antonio is unable to pay back the loan. Note too that after Shylock's reference to Jacob's gulling of his father in law, Laban, Antonio compares Shylock to the Devil in his ability to twist scripture to make it say what he wants it to say.
Clearly, therefore, both Shylock and Antonio have a deep and bitter hatred of each other, based on Antonio's mistreatment of Shylock. I think that it is particularly interesting that Antonio chooses to punish Shylock in the most profound way possible at the end of the story - Antonio knows that Shylock would prefer death than conversion, and it is a very cruel ending which should give the audience sympathy for Shylock.
I would add that perhaps a better brief summary of the matter is "I never heard a passion so confused, / So strange, outrageous, and so variable"(2.8.12-13). In Shylock's aside(1.3) we find it is ended with "Cursed be my tribe / If I forgive him." One might say he feels Antonio should suffer some penalty for his offences, and Antonio does spend time in jail. In Act 1., scene 2 Portia says "If to do were as easy as to know what were good to do" etc. Shylock also confronts Antonio with "I would be friends with you and have your love" etc(1.3.134). Of Antonio we read "A kinder gentleman treads not the earth"(2.8.35). Antonio says: "Content, in faith. I'll seal to such a bond, / And say there is much kindness in the Jew."(1.3.149-150). I have here and there, partially documented that MV is linked throughout with ROMEO AND JULIET. Romeo says to Tybalt: "Mercutio's soul / Is but a little way above our heads, / Staying for thine to keep him company. / Either thou or I, or both, must go with him"(ROM3.1.126-129). In the court scene in MV, it might seem that Antonio and Shylock are contending who gets to join Leah "in heaven"(4.1.225).